# transistor basics

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#### neo_star

##### New Member
hi
i am a beginer and need some help.

Wat is a transistor?
Wat is biasing of a transistor?
and Wat is Q point of a transistor?

#### Gayan Soyza

##### Active Member
What is a Transistor?

A transistor is an electronic component used in a circuit to control a large amount of current or voltage with a small amount of voltage or current given to it.

What is Biasing of a Transistor?

Biasing is the process of applying electrical energy across the transistor in order to make it operate as we require.

What is Q Point of a Transistor?

The q-point is the DC operating point of the transistor. It is also known as the bias point. When setting a transistor to a q-point you are biasing the transistor to a certain voltage and/or current in order for the transistor to operate in a desired way.

#### Mikebits

##### Well-Known Member
What is a transistor?

It is a cross dressing resistor.

What is biasing point of transistor?

The point at which the transistors decision will be based, for or against.

What is Q point of transistor?

A measure of when a transistor will decide to answer a question.

##### Banned
The Q point of this board is negative =\

#### Willbe

##### New Member
A transistor is an electronic component used in a circuit to control a large amount of current or voltage with a small amount of voltage or current given to it.
Actually a transistor [can be modeled as] a current controlled current source. The voltage stuff comes out because xxxxxxxxxx.

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##### Banned
Actually willbe, you have it backwards =) On an atomic scale the transistor is a voltage controlled device, the current is only because the junctions aren't isolated. This is however merely a technicality, you'll never see it taught that way in school. Unless perhaps you're going into advanced semi-conductor theory.

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#### Claude Abraham

##### Member
Actually willbe, you have it backwards =) On an atomic scale the transistor is a voltage controlled device, the current is only because the junctions aren't isolated. This is however merely a technicality, you'll never see it taught that way in school. Unless perhaps you're going into advanced semi-conductor theory.
No you have it wrong. On an atomic scale a bjt is a charge controlled device as is a FET. The bjt is a minority carrier device, whereas FETs are majority carrier. On the macro scale, bjt is current controlled, and FET is voltage controlled.

Every semiconductor maker and university in the world knows and acknowledges this.

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#### Roff

##### Well-Known Member
No you have it wrong. On an atomic scale a bjt is a charge controlled device as is a FET. The bjt is a minority carrier device, whereas FETs are majority carrier. On the macro scale, bjt is current controlled, and FET is voltage controlled.

Every semiconductor maker and university in the world knows and acknowledges this.
I'm sure you know the well-known relationsip between Vbe and emitter current (the diode equation). This relationship (voltage control mode) can be very useful, e.g. in making a logarithmic amplifier.

Also see http://www.electro-tech-online.com/custompdfs/2009/04/log104.pdf

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##### Banned
SCIENCE HOBBYIST: how transistor works, an alternate viewpoint

Claude, you're mistaking 'charge controlled' to mean current controlled. Current is not required to flow for the semi conductor effect to exist. The fact that current has to flow in a BJT for it to function is only because of it's design, the voltage field itself is what is actually doing the work to switch the semi conductor's conduction state. It's easiest to visualize if you only look at a simple PN junction, it's the voltage causing the change in the depletion layer that causes current to flow in the diode. ALL semi conductor devices (SCR's BJT's FET's) work the same way, due to their internal construction though you get all the varried effects you see from different devices.

Again, this is completely academic and not required information to understand or use transistors in the real world because the model of the transistor as a current controlled device is more logical to the way it actually functions, do not mistake this for the REASON for it to function though.

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#### Claude Abraham

##### Member
SCIENCE HOBBYIST: how transistor works, an alternate viewpoint

Claude, you're mistaking 'charge controlled' to mean current controlled. Current is not required to flow for the semi conductor effect to exist. The fact that current has to flow in a BJT for it to function is only because of it's design, the voltage field itself is what is actually doing the work to switch the semi conductor's conduction state. It's easiest to visualize if you only look at a simple PN junction, it's the voltage causing the change in the depletion layer that causes current to flow in the diode. ALL semi conductor devices (SCR's BJT's FET's) work the same way, due to their internal construction though you get all the varried effects you see from different devices.

Again, this is completely academic and not required information to understand or use transistors in the real world because the model of the transistor as a current controlled device is more logical to the way it actually functions, do not mistake this for the REASON for it to function though.
I am not mistaking "charge" control" for "current control". I know the difference. This may come as a shock to you, but there are lots of people in this world who know semiconductor physics. The experts work for Fairchild, On Semi, Texas Instr, etc. Every semi maker classifies both FETs and bjts as charge controlled at the mIcro level. The basic difference is that the bjt is a minority carrier device, and the FET is majority carrier. Every semiconductor physics text concurs.

At the mAcro level, the FET is voltage controlled and the bjt is current controlled.

In the simple pn junction diode case, the voltage across the depletion layer is NOT the "cause" of the current. How does the depletion layer originate?

With no external bias, electrons and hole pairs are thermally generated. The carriers receive energy from the vibrations in the lattice. In quantum mechanics, QM, these packets of acoustic energy are called phonons. When the carriers cross from p to n & n to p type material, they recombine. But a finite time is needed, called minority carrier lifetime.

Now when an external bias is provided, the electric field results in drift current. But this electric field is a result of the external source, battery, or generator. The chemical energy conversion (battery) or mechanical energy conversion (generator) sets up the E field. The holes and electrons easily transit through their p & n materials, as they are highly mobile being majority carriers.

But when they cross the junction, they have much less mobility, as they now become minority carriers, In a matter of time, carrier statistical lifetime, they recombine. So, in a p-n junction, there is a distribution of minority charge separated by a layer void of charges, called depletion region, or "space charge" region. The integral of the associated E field along the path from edge to edge is by definition the voltage.

The voltage and current, are both produced by the battery or generator, i.e. the external power source providing the work/energy. In order to increase the junction voltage, the rate of carrier flow must be increased. At higher values of forward current, the minority charge built up in the depletion region increases. The E field increases as well due to the increased charge concentration. The voltage also increases.

In order to increase Vf, the forward voltage drop of the p-n junction, the current must be increased. The increase in current requires that the external source deliver more power. The increase in Vf takes place after the increase in If (forward current). Vf is NOT the "cause" of If, NOR VICE-VERSA.

Seting up a circuit in the lab with a reasonably good scope and signal generator will affirm the above. A change in base current precedes a change in collector/emitter current. At low frequencies it may be hard to see, but at high frequencies it's visible.

The current in a p-n junction is NOT caused by the voltage across said junction. That is one thing that you must be willing to examine. It's hard to question something you have believed in for years. But I once thouight along the same lines as you. I used to view voltage as the cause of current. But e/m field theory, and working in the electronics field convinced me otherwise. Peace.

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#### Claude Abraham

##### Member
I'm sure you know the well-known relationsip between Vbe and emitter current (the diode equation). This relationship (voltage control mode) can be very useful, e.g. in making a logarithmic amplifier.

Also see http://www.electro-tech-online.com/custompdfs/2009/04/log104-1.pdf
Just because there is a functional relationship between Vbe & Ie does not mean that a bjt should be controlled by directly varying Vbe. The collector current can be expressed as a math function of any of 3 input variables.

Ic = beta*Ib

Ic = alpha*Ies*[exp((Vbe/Vt)-1)]

Ic = alpha*Ie

The base & emitter terminals are the control electrodes. So the question becomes, do we control Ic by controlling Ib, Vbe, or Ie.

WIth Ib, controlling Ic is difficult due to what is known as "beta dependency". In general, we don't use Ib to control Ic, except for switch applications.

With Vbe, controlling Ic is also very difficult since Ies, the reverse saturation current of the b-e junction is difficult to control. Also, the Ies value increases with temp. If a constant 0,65 or 0.7 volt source is connected across b-e, current is established resulting in a temperature increase. But Ies increases with temp. So Ib increases more, further increasing Ies, etc. THis is thermal runaway.

With Ie, the relation Ic = alpha*Ie, is very precise. Whereas beta can drop to 50 and reach 500 due to device variations and temp, alpha is 0.99 plus or minus 0.01. Also, controlling Ic with Ie is thermally stable, no runaway.

This is why the bjt is classified as current controlled. Moreover, the "current controlled" refers usually to *emitter*, NOT base current. We control Ic w/ emitter current.

Regarding log amps, please refer to U.S. patent no. 5,670,775, which I received in 1997, for a unique log amo used w/ photodiodes. I am well aware of the logarithmic I-V relation in diodes. Cheers.

##### Banned
Every semi maker classifies both FETs and bjts as charge controlled at the mIcro level
Charge = difference in the number of electrons/holes between two areas, this is voltage not current. It's is the STATIC charge that determines the state of a semi conductor junction. Charge is NOT current.

Now when an external bias is provided, the electric field results in drift current. But this electric field is a result of the external source, battery, or generator. The chemical energy conversion (battery) or mechanical energy conversion (generator) sets up the E field.
You used electric field to to describe what sets up the drift currents, again showing that it's the charge that's causing the current to flow.

Seting up a circuit in the lab with a reasonably good scope and signal generator will affirm the above. A change in base current precedes a change in collector/emitter current. At low frequencies it may be hard to see, but at high frequencies it's visible.
I didn't say otherwise, those currents are inherent in the structural design of a BJT heterojunction, the currents themselves however are NOT what cause the semi conductor to change conduction states it is in fact that Vbe that determines how a transistor functions, the currents that results from teh Vbe being forward biased are incidental to the reason for it's functioning even if they are what make it useful.

The depletion region in a semi conductor PN junction IS the cause of the voltage imballance within the junction.
The p-n junction possesses some interesting properties which have useful applications in modern electronics. A p-doped semiconductor is relatively conductive. The same is true of an n-doped semiconductor, but the junction between them is a nonconductor. This nonconducting layer, called the depletion zone, occurs because the electrical charge carriers in doped n-type and p-type silicon (electrons and holes, respectively) attract and eliminate each other in a process called recombination. By manipulating this nonconductive layer, p-n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction. This property is explained in terms of the forward-bias and reverse-bias effects, where the term bias refers to an application of electric voltage to the p-n junction.
I see not one mention of a PN junction being controlled by a current anywhere.

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##### Banned
Transistor Operating Details
Please note the second box "Collector current determination"

Again, just because the common model of a transistor displays it as a current operated devices does NOT mean they in fact are, it's just the effect that makes them useful as devices. You I'm afraid are the one that has been mis informed =)

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#### Willbe

##### Banned
I'm just slightly confused why on multiple occasions in Claude's post he says that they're charged control devices and then proceeds to say that they're current controlled devices.

#### Willbe

##### New Member
I'm just slightly confused why on multiple occasions in Claude's post he says that they're charged control devices and then proceeds to say that they're current controlled devices.
"One coulomb is the amount of electric charge transported in one second by a steady current of one ampere."

I guess he's timeless, but only at times.

A few deciliters of rum seems to improve my writing skills!

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#### Claude Abraham

##### Member
Charge = difference in the number of electrons/holes between two areas, this is voltage not current. It's is the STATIC charge that determines the state of a semi conductor junction. Charge is NOT current.

You used electric field to to describe what sets up the drift currents, again showing that it's the charge that's causing the current to flow.

I didn't say otherwise, those currents are inherent in the structural design of a BJT heterojunction, the currents themselves however are NOT what cause the semi conductor to change conduction states it is in fact that Vbe that determines how a transistor functions, the currents that results from teh Vbe being forward biased are incidental to the reason for it's functioning even if they are what make it useful.

The depletion region in a semi conductor PN junction IS the cause of the voltage imballance within the junction.
I see not one mention of a PN junction being controlled by a current anywhere.
You claim that charge is the difference between 2 areas, and then proceed to call it voltage. Voltage is the WORK per unit charge expended in creating that separation. Your definitions are circular. You accuse me of not knowing the difference between charge & current, and then you treat charge & voltage as being the same thing.

To separate charges on each side of the depletion region, work had to be done transporting them. This work takes finite time. Thus if energy is changing, as work is being done, dw/dt is non-zero. But dw/dt is power & P = I*V. Hence to separate charges and create a depletion layer, I & V must BOTH be non-zero. That voltage associated with separation of charge carriers is not possible without the movement of said carriers which is by definition current.

Then you say that the depletion region is the CAUSE of the voltage imbalance within a junction. Please define what you mean by "voltage imbalance". Do you mean "charge imbalance"? Honestly I'm not being difficult. The depletion region is a void. How can it be the cause of anything. Please reread my post involving thermal generation of ehp (electron-hole pairs). Energy, either intrinsic thermal, or external source, is imparted to charge carriers, moving them towards the junction. When that junction is crossed, the carriers encounter a great decrease in mobility, as they quickly become minority carriers. They have a ststistical lifetime before recombination occurs. So there is a distribution of charges at the edges of the junction depletion layer. Of course, there is an associated voltage. Separated charges must have an associated E field. The line integral of said E field over the path of the layer is the voltage. Even w/o an external bias, there is a built in potential, Vbi. But this potential in either case is NOT what drives anything. The thermal energy or external power source is what drives the above phenomenon.

Finally, you keep asking me "how can this behavior be caused by a current, no reference says that current is the cause". I've stated repeatedly that current is NOT the cause. On a micro scale, the device is charge controlled. On the macro scale, when I describe the device as current controlled, no causality is implied. Just as the forward voltage in a p-n junction, Vf, does not cause forward current, If, the converse is equally true. If is NOT the cause of Vf. Whan you keep reiterating that semiconductor behavior is not caused by current, you are atacking a straw man you yourself created. Nowhere did I imply that current causes anything.

Rather, I said explicitly that the best way to control Ic is to control Ie. When I say Ic = alpha*Ie, I do not imply causality. If we establish a fixed current value in the emitter, the collector current is very predictable and reliable. With temp variations, operating current level, and device variations, alphs ranges in value from 0.98 to 0.998. Thus referring to a bjt as "current controlled" merely implies that the result obtained by establishing Ie and then relying on alpha to get Ic is very precise.

However, establishing an Ib value, and then relying on beta is a bad practice. You can do it, but the circuit becomes "beta dependent". For a given vaslue of Ib, the resulting value of Ic is widely variable. Likewise if we establish a fixed value of Vbe, the value of Ic is wild and unpredictable. If I tell you that a certain bjt has Vbe = 0.65V, you really have no idea what Ic is. It could be microamps, milliamps, or even amps. Attempting to control Ic via Vbe is not viable. That is why we don't attempt to control bjt with voltage. Hence we refer to it as "current controlled". It has nothing to do with causality.

Then you mention that the currents are "incidental" wrt junction voltage. But junction voltage is also incidental. When forward current is established, carriers build up at the depletion layer edges. Vf & If are mutually exclusive. You are wrong insisting that Vf causes If. You are right insisting that If does not cause Vf.

The "cause" of electric, magnetic, mechanical, thermal, etc. events is the transfer of energy. I think in terms of charge and energy as they are immutable and always conserved. Current and voltage are not. Have I explained it well?

Claude

#### Mr RB

##### Well-Known Member
So...

Wat is a transistor????

Hehe. Just kidding.

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